1. Field of the Invention
The present invention relates to a method of determining fires in which outputs from a plurality of types of fire sensors monitoring different objects are processed in a manner in which the outputs are combined to detect the outbreak of fires and to give an alarm. More particularly, this invention pertains to a combined method of determining fires in which a plurality of thresholds are set at various types of sensors, and the outputs from the sensors are processed in a combined manner, thereby improving the accuracy of determining the outbreak of fires.
2. Description of the Related Art
FIG. 12 illustrates a fire determining system to which a conventional method of determining fires is applied. In this system, a plurality of sensors 1a-1n arranged at appropriate zones to be monitored are connected to a signal receiving device 2 through a signal transmission line. The device 2 continually receives signals transferred from the sensors, and thereby determines whether or not a fire has occurred. Once the signal receiving device 2 determined that a fire has occurred, it starts alarm devices 3, such as alarm ringing devices, and actuates fire-preventing equipment 4, such as fire doors, smoke dispersion preventing devices and automatic fire-extinguishing devices.
It is possible to employ the following sensors: sensors for determining fires on the basis of a rise or change in temperature or in the smoke density in the air. Such sensors include a so-called fixed-temperature heat sensor which generates signals when the temperature of the air exceeds a preset threshold; a differential heat sensor which monitors the ratio at which air temperature increases and generates signals when this ratio exceeds a preset ratio; and a smoke sensor which generates signals when the smoke density in the air exceeds a preset threshold.
The conventional fire determining method, to which the above sensors are applied, has the drawback of a so called false alarm, that is, when there is actually no fire, it determines that a fire has broken out, and sets out an alarm. FIG. 13 shows the results of investigating the actual conditions in which false alarms (without a fire) were given between 1980 and 1981 ("the results of investigating the actual conditions in which automatic fire alarm equipment sets out false alarms" by Tokyo Fire Defense Agency). FIG. 14 shows the results of analyzing the causes of false alarms on the basis of the above investigation. As obvious from the results shown in FIG. 13, six false alarms are sent from 1000 heat sensors, whereas six false alarms are sent from 100 smoke sensors. The incidence of false alarms from the smoke sensors is a problem compared with that of the heat sensors. As apparent from FIG. 14, these false alarms are rarely given because of the failure of equipment, such as the sensors, but mostly because of misreading man-made causes, such as smoke from cooking or cigarette.
To clarify the causes of false alarms from smoke sensors, the inventor of this invention empirically investigated the relationship between the sensitivity of smoke sensors and the magnitude of fire (heat release values). FIG. 15 shows the results of this investigation. For each burning method and material burned, the heat release value of the fire source is given under conditions where a photoelectric smoke sensor is provided on a 3-m high ceiling, and the fire source is provided on a floor surface. As the results of the investigation indicate, when the heat release value of the fire source is regarded as a criterion, the photoelectric smoke sensor has extremely high sensitivity to fires in a smoldering state; for example, it absolutely detects a small fire in the smoldering state at a level of 0.16 kW.
The sensitivity of photoelectric smoke sensors to fires in a flaming state varies greatly according to the type of material burned. The sensitivity of the photoelectric smoke sensor is higher than that of a differential heat sensor to a fire of a material, such as polyurethane, which produces a great amount of smoke. On the other hand, the sensitivity of the photoelectric smoke sensor is lower than that of the differential heat sensor to a fire of a material, such as timber, which produces a small amount of smoke.
Even when a fire source with a heat release value corresponding to 0.16 kW is placed, it is rare for smoke to rise to the ceiling because the temperature in an air stream is low. In other words, a heat source is required for generating an air stream which sends smoke up to the ceiling. If a temperature of 2 (deg) is required for the air stream to reach the ceiling, a heat release value required for such a rise in temperature is approximately 2.5 kW. The photoelectric smoke sensor (first type) operates under the conditions, using the above values, where the height of the ceiling is 3 m, a heat source corresponding to 2.5 kW and a smoke source corresponding to 0.16 kW smoldering are disposed on the floor surface. However, there are innumerable man-made occasions meeting such conditions. For instance, the combination of steam and heat from a heating system or of heat from a heating system and cigarette smoke, or smoke produced during cooking, welding, etc. in daily life. The photoelectric smoke sensor may thus be actuated in some cases depending on the conditions, even if a fire has not occurred.
By merely detecting smoke as a product of burning, limitations are established for distinguishing a real fire from a similar, man-made phenomenon. Originally, smoke sensors have an advantage of high sensitivity for detecting a smoldering state in an early stage of a fire. These smoke sensors, however, have the disadvantage of a high incidence of false alarms. As understood from FIG. 15, heat sensors have a characteristic of responding to the magnitude of a fire source (heat release value). However, there is a limit to the sensor's detection capability depending on the magnitude of the fire source.